Kadir Every account (subaccount) at bunq has its own IBAN. (The IBAN is displayed at the top of every subaccoubt) You can transfer money to this IBAN from ING. When you only have the Travelcard membership, you don't have accounts, but in that case you get an IBAN to top up your card.
You can also add money using iDEAL, another card, sofort....
When you go to the page 'Home', press the "MENU" button, and choose "Add Money", you see all the options to add money.